Potato (Solanum tuberosum L.) is the world's fourth most valuable crop. In the United States of America, the value of the crop exceeds two billion dollars each year. Worldwide production of the cultivated potato exceeds that of all other dicot food crops. Potato is also host to more than sixty pathogens of economic significance, causing costly diseases in terms of crop loss. The expenses associated with application of chemicals, and the environmental impact of pesticide use, are significant. Such costs could be minimized or avoided if resistant potato varieties were available. However, adequate resistance for many diseases has not been incorporated into potato cultivars, partly because of the lack of resistance genes that breeders can use to develop resistant cultivars.
Among the most devastating potato diseases is late blight, a foliar and tuber disease caused by the oomycete pathogen Phytophthora infestans (P. infestans), the causative agent of the legendary Great Irish Potato Famine of 1845. The late blight fungus is also a devastating pathogen on crops other than potato; it infects tomatoes, eggplants, and other solanaceous species. To combat the disease caused by P. infestans, growers use a combination of practices, such as sanitary measures, resistant cultivars, and fungicides. The fungicide approach has repeatedly failed due to the remarkable ability of P. infestans to acquire resistance. The attempted breeding of disease resistant strains of Solanum tuberosum (S. tuberosum), the cultivated species of potato, has also failed over time.
Possible sources of resistance to many potato pathogens exist in the approximately 225 wild Solanum species. Several Solanum species have been crossed with the cultivated potato in an effort to introgress disease-resistance genes, including genes that confer resistance to late blight disease (Jansky, 2000, Plant Breed. Rev. 19: 69-155). Among wild potato species with late blight resistance is the hexaploid Solanum demissum. Resistance from this species has been incorporated into potato via sexual crosses. Eleven race-specific resistance genes conferring late blight resistance have been described in Solanum demissum (Malcolmson and Black, 1966, Euphytica 15:199-203), and introgressed into cultivated potato varieties using classical breeding. These genes are characterized by pathogen race specificity and a hypersensitive phenotype. Unfortunately, virulent races of P. infestans have rapidly overcome the majority of these 11 late blight resistance genes in most potato growing regions (Fry and Goodwin, 1997, Plant Disease 81: 1349-1357).
Because P. infestans is capable of acquiring resistance, efforts have been directed toward the identification of additional late blight resistance genes in wild potato species that are naturally resistant to P. infestans. For example, Rpi1, a late blight resistance gene from Solanum pinnatisectum, was described and mapped by Kuhl et al., 2001, Mol. Genet. Genomics 265: 977-985. Rpi1 has never been deployed for potato protection and the durability potential of Rpi1 remains unexplored. In addition, to confer late blight resistance, somatic hybrids between cultivated potato and the wild Mexican diploid Solanum bulbocastanum (S. bulbocastanum) have also been generated. Such somatic hybrids retained the late blight resistance of the wild species, and could be backcrossed to cultivated potato (Helgeson et al., 1998, Theor. Appl. Genet. 96: 738-742). Mapping experiments revealed a single locus on Solanum bulbocastanum chromosome 8 that imparted the late blight resistance phenotype (Naess et al., 2000, Theor. Appl. Genet. 101: 697-704). This region was dubbed RB (resistance region from S. bulbocastanum), and a gene from S. bulbocastanum that confers late blight resistance is referred to as RBblb.
The global food shortage crisis has highlighted the importance of the ongoing quest for materials and methods for conferring disease resistance in plants, for example as disclosed in International Patent Application Publication No. WO/1999/009151, and in particular the quest for potato genes for resistance to late blight, as disclosed in U.S. Patent Application No. 2005/0204419 A1. However, despite decades of active breeding effort to control late blight, this disease still causes the loss of billions of revenue dollars for growers each year (Kamoun, 2001, Curr. Opin. Plant Biol. 4: 295-300). Accordingly, a source of resistance to Phytophthora species that could be introduced into the cultivated species by molecular genetic techniques would be of great value. As a result, there is an ongoing need to identify genes that might confer late blight disease resistance. If such genes can be identified and isolated, they can be introduced by molecular genetic techniques into domestic potato and species other than potato to confer resistance to one or more plant pathogens. The products of such research are in demand by potato growers, who keep looking for novel varieties containing genes and other factors that promote resistance to P. infestans and related pathogens. The present invention addresses these and other related needs.